The use of enzymes to improve the chemical, physico-chemical or organoleptic nature of food grade products is wide spread. Also in the processing of cow milk and other animal derived substrates, the use of enzymes adds significant value to the end product. Examples are incubations with lactase to render milk acceptable for lactose intolerant individuals, proteolytic hydrolysis of casein and whey proteins to alleviate allergenicities and to improve foam characteristics, the modification of egg phospholipids using phospholipase A2 to improve baking performance and stabilize mayonaises, the use of transglutaminases on meat and fish products to improve hardness and elasticity as well as the removal of oxygen from egg products or grated cheese by adding glucose oxidase. Additionally, enzyme treatments are being used to enhance the flavor of various animal derived food products. For example, proteases are being used to speed up flavour development in fish and meat extracts. Furthermore, accelerating flavor development in cheese is a well known target. Whereas EMC (Enzyme Modified Cheese) is an established product in which primarily various lipases are used, speeding up the subtle taste changes involved in the aging of cheeses by adding minor quantities of exoproteases, lipases or esterases, is a more recent development.
The invention also relates to lactase. Lactase or β-galactosidase (E.C: 3.2.1.23) is an enzyme, which catalyzes the hydrolysis of lactose (a disaccharide) into its component monosaccharides glucose and galactose. Lactose is present in dairy products and more specifically in milk, skimmed milk, cream and other milk products. The breakdown of lactose occurs in the intestinal wall of the human body (and other mammals) by the natural presence of lactase.
The nutritional and functional problems caused by lactose in most populations that lack lactase are well known and described. Members of such populations cannot hydrolyze the lactose, which in such cases passes into the large intestine, where it produces dehydration, poor calcium absorption, diarrhea, flatulence, belching and cramps, and, in severe cases, even watery explosive diarrhea.
An important industrial application of lactase is in the production of lactose-hydrolyzed milk products for lactose intolerant individuals. Such hydrolysed milk products include pasteurized milk, UHT-milk and milk reconstituted from all or part of its original constituents with or without intermediate processing steps such as protein hydrolysis. Treatment with lactase may be done prior to and after the heat-treatment of the milk. The lactase treatment may be done by adding the enzyme to the milk. The solubility properties of lactose are such that it may lead to its crystallization, leading to a sandy or gritty texture. Such undesired texture may be found in some dairy products such as condensed milk, evaporated milk, dry milk, frozen milk, ice cream, and in confectionery products with a high content of milk. Full or partial hydrolysis of lactose by lactase eliminates this problem, providing products with a homogeneous texture and as a result a higher consumer acceptance.
Another industrial application of lactase is to increase sweet taste in lactose containing products like milk or yoghurt. The hydrolysis of lactose in such products results in increased sweet taste as a result of the production of glucose. Another industrial application of lactase is the hydrolysis of lactose products containing dairy components such as bread. Lactose is added in such products to enhance flavour, retain moisture, provide browning and improve toasting properties. Hydrolyzed lactose syrups are promising in terms of e.g. enhancing crust-colour development, improving flavour and aroma, modifying texture, extending shelf life and strengthening loaf structure.
Lactose hydrolysis by lactase in fermented milk products such as yoghurt will increase sweet taste. However, when the lactase is added prior to the beginning of the fermentative process, it may increase the rate of acid development and thus reduce processing times. The lactase treatment of milk or milk-derived products such as whey makes such products suitable for application in animal feed and pet food for lactose intolerant animals such as cats. The lactose hydrolysis allows the manufacture of a higher concentrated whey and at the same time prevents gut problems, similar to those described earlier for lactose-deficient patients. Lactose hydrolyzed whey is concentrated to produce a syrup containing 70-75% solids and is used as a food ingredient in ice cream, bakery and confectionery products.
Lactases have been described for and isolated from a large variety or organisms, including micro-organisms. Lactase is often an intracellular component of micro-organisms like Kluyveromyces and Bacillus. Kluyveromyces and especially K. fragilis and K. lactis, and other yeasts such as those of the genera Candida, Torula and Torulopsis are a common source of yeast enzymes lactases, whereas B. coagulans or B circulans are well known sources for bacterial lactases. Several commercial lactase preparations, derived from these organisms are available such as Maxilact® (from K. lactis, produced by DSM, Deflt, The Netherlands). All these lactases are so called neutral lactases since they have a pH optimum between pH=6 and pH=8. Several organisms such as Aspergillus niger and Aspergilus oryzae can produce extracellular lactase, and U.S. Pat. No. 5,736,374 describes an example of such lactase, produced by Aspergilllus oryzae. The enzymatic properties of lactases like pH- and temperature optimum vary between species. In general, however, lactases that are excreted show a lower pH-optimum of pH=3.5 to pH=5.0; intracellular lactases usually show a higher pH optimum in the region of pH=6.0 to pH=7.5, but exceptions on these general rules occur. The choice for a neutral or acidic lactase depends on the pH profile in the application. In applications with neutral pH, neutral lactases are usually preferred; such applications include milk, ice cream, whey, cheese, yoghurt, milk powder etc. Acid lactases are more suited for applications in the acidic range. The appropriate lactase concentration is dependent on the initial lactose concentration, the required degree of hydrolysis, pH, temperature and time of hydrolysis.
Although aimed at improving the functionality and/or taste profiles of the food product, occasionally an enzyme treatment can have unexpected and undesirable side effects. An example of an undesirable side effect is the development of off-flavor as a result of the enzyme treatment.
Mettall et. al, The Australian Journal of Dairy Technology, (1991), 46-48 describes the problem of off-flavor development when milk is treated with lactase. According to this publication high levels of protease will result in the rapid development of off-flavors. Production processes are therefore optimised to minimize proteolytic side activities in order to reduce the risk of off-flavour formation. An example of a purification process for K. lactis derived lactase is described in WO 02/081673.
It is found that even lactase preparations with low protease activity can still give rise to off-flavour formation. This is especially the case for the neutral lactases, derived from the cytoplasm of yeast. The off-flavour formation that is associated with the use of lactase preparations is especially critical for lactose hydrolysed UHT-milk. The lactases that are used in this case are neutral lactases because of their favourable pH optimum for milk. The UHT milk has received a high heat treatment to obtain a shelf life of several months at room temperature. The long storage times outside the refrigerator make these products especially prone to off-flavour formation: even a very low off-flavour formation rate can give rise to significant off-flavour formation after several months of storage, making the product unattractive for consumption.